Unlike Qt5, the Qt4 signals implementation doesn't allow a signal to be
directly connected to a c++ lambda expression. Work around this by defining a
Callback QObject with a virtual method that can forward calls to a closure.
The Qt4 error was reported by Patrick Strateman <patrick.strateman@gmail.com>
in https://github.com/bitcoin/bitcoin/pull/10039#issuecomment-289248763
Construct scoped_connection directly instead of relying on copy initialization
and move constructor. Avoids the following compile error in debian jessie:
```
In file included from /usr/include/boost/signals2/signal.hpp:21:0,
from ./util.h:29,
from ./dbwrapper.h:11,
from ./txdb.h:10,
from ./test/test_bitcoin.h:11,
from qt/test/wallettests.cpp:11:
/usr/include/boost/signals2/connection.hpp: In function ‘uint256 {anonymous}::SendCoins(CWallet&, SendCoinsDialog&, const CBitcoinAddress&, CAmount)’:
/usr/include/boost/signals2/connection.hpp:234:7: error: ‘boost::signals2::scoped_connection::scoped_connection(const boost::signals2::scoped_connection&)’ is private
scoped_connection(const scoped_connection &other);
^
qt/test/wallettests.cpp:47:6: error: within this context
});
^
```
Error reported by Pavel Janík <Pavel@Janik.cz> in
https://github.com/bitcoin/bitcoin/pull/9974#issuecomment-287550034
